Electromagnetic coal seam gas recovery system

ABSTRACT

A system for recovering gas trapped within the earth includes a casing ( 24 ) sized and configured to be positioned within a borehole in the earth, the casing ( 24 ) formed of a material that is transmissive to electromagnetic energy and gas within the earth; an antenna ( 40 ) sized and configured to be positioned within the casing ( 24 ). The antenna ( 40 ) has a distal end and a proximal end and including a radiating element at the distal end of the antenna ( 40 ) which, in operation, transmits electromagnetic energy toward a desired area of the earth, and an interior channel for allowing gas to be conveyed from the distal end to the proximal end of the antenna ( 40 ).

BACKGROUND

[0001] The invention relates to the recovery of gas from subterraneanformations in the earth.

[0002] Extensive and high volumes of hydrocarbon gases (e.g., methane)trapped within coal seams have been discovered in various parts of theUnited States. For example, large amounts of trapped methane gas havebeen discovered in eastern Wyoming (see, for example, “Powder RiverBasin Coalbed Methane Play Heats Up,” E&P Perspectives, Vol. X, R57,Oct. 22, 1998 (attached herewith). Naturally occurring degradationprocesses, such as the biodegradation of microorganisms in the coal isbelieved to cause the generation of the methane gas trapped within thecoal seams.

[0003] Methods of economic and environmentally sound gas recovery areunderway. A major problem encountered is the large amount of aquifers(water) that impedes the ability to recover the gas from bore holesdrilled in to the coal seam. Specifically, the in-ground water serves asa barrier to the effective removal of the gas from the bore hole. Thewater must be removed by a pump or redirected to allow more efficientremoval of the gas. Systems of co-generation of power for pumps arebeing considered for the prime supply of electrical energy for thepumps. That is, the electrical power for operating gas turbines used todrive the pumps could be generated using a portion of the gas removedfrom the borehole.

SUMMARY

[0004] In a general aspect of the invention, a system for recovering gastrapped within the earth, the system includes a casing sized andconfigured to be positioned within a borehole in the earth, the casingformed of a material that is transmissive to electromagnetic energy andgas within the earth, and an antenna sized and configured to bepositioned within the casing. The antenna includes a radiating elementat a distal end of the antenna which, in operation, transmitselectromagnetic energy toward a desired area of the earth, and aninterior channel for allowing gas to be conveyed from the distal end toa proximal end of the antenna

[0005] In another aspect of the invention, a method system forrecovering gas trapped within the earth includes the following steps. Acasing is positioned within a borehole in the earth, the casing formedof a material that is transmissive to electromagnetic energy and gaswithin the earth. An antenna is positioned within the casing, theantenna having a distal end and a proximal end. The antenna includes aradiating element at the distal end of the antenna which, in operation,transmits electromagnetic energy toward a desired area of the earth; andan interior channel for allowing gas to be conveyed from the distal endto the proximal end of the antenna The method further includes applyingelectromagnetic energy to the antenna to radiate the earth surroundingthe casing; drawing the gas within the earth into the interior channelof the antenna at the distal end of the antenna; and conveying the gaswithin the interior channel to the proximal end of the antenna.

[0006] Embodiments of these aspects of the invention may include one ormore of the following features.

[0007] A product return pipe has a first end connected to the proximalend of the antenna and a removable cap attached to a second end of theproduct return pipe. A bellows is connected to the proximal end of theantenna A thermocouple assembly is connected to the proximal end of theantenna.

[0008] The antenna is configured to operate in a frequency range between300 KHz and 300 GHz. More particularly, the frequency range is between 1MHz and 100 MHz (e.g., about 27 MHz). The antenna is configured tooperate at a power level in a range between 3 Kwatts and 20 Kwatts(e.g., about 10 Kwatts).

[0009] Among other advantages; the system and method reduces thenegative impact of water on the in situ recovery of coal gas, such asmethane from underground beds or seams of coal; and (2) to provideadditional or enhanced stimulation of gas production from the coaldeposits.

[0010] The basic energy source proposed for reducing the water barriereffect and stimulating production in-situ is electromagnetics.Electromagnetic energy at frequencies as low as 60 Hz and extending intoto microwave frequencies supplied by earth electrodes in the form ofantennas and/or waveguides may be employed in the proposed processes.The basic idea is to introduce current into the subterranean formationto vaporize or boil the water in a specified region or of the coal seam.The currents are derived from the electromagnetic field energy absorbedby the coal material and water.

[0011] Specific in-ground applicator structures such as rod electrodes,antennas or waveguides and transmission lines provide the inducedcurrents in the coal seam to vaporize a given amount of water. Forexample, antennas in a vertical or horizontal bore hole drilled in acoal seam radiate electromagnetic energy away from the antenna into thecoal creating a dry region around the bore hole/antenna structure. Apump can be used in conjunction with the antenna for water removal orthe bore hole containing the antenna may be pressurized to keep thewater away from the antenna/bore hole.

[0012] A special gas filtering system can be employed around the antenna(within or outside the bore hole) to permit gas recovery up the antennabore hole without water. This special filter would block liquid waterand allow only gas to pass through it. The dry region around the antennaborehole created by dielectric heating of the coal/water matrix ismaintained by the power supplied by the antenna (e.g.. 3 to 20 kilowattson average).

[0013] This dry region, maintained by either resistive (low frequency)currents or dielectric (high frequency) currents in the coal seam,allows the gas to be transferred from regions outside the casing towithin the antenna case, bore hole, or adjacent recovery wells equipmentwith special filters and flow lines for ease of gas recovery withoutwater.

[0014] The dry sheath region or zone is maintained at approximately 100°C. to ensure that there is no liquid water.

[0015] Thermal energy is not a requirement for the gas deposits inplace. As a result of the dielectric sheath created by electromagneticcurrents, the radiation fields of the antenna now extend further intothe coal seam away from the antenna bore hole thereby creating anenhanced zone or region of heating and results in an enlargement of thedry zone and less impedance of gas flow to the recovery well by water.

[0016] Another benefit of electromagnetic heating is the enlargement offracture zones in the coal seams by stream pressure and thermalgradients. The result is enhanced flow of methane gas to recovery wells.

[0017] Still another benefit of electromagnetic heating is the increasedactivity of microorganisms from the thermal energy deposit, especiallyat radio frequencies.

[0018] The details of one or more embodiments of the invention are setforth in the accompanying drawings and the description below. Otherfeatures, objects, and advantages of the invention will be apparent fromthe description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

[0019]FIG. 1 illustrates the upper portion of an RF gas recovery systemin accordance with the invention.

[0020]FIG. 2 illustrates the lower portion of the RF gas recovery systemof FIG. 1.

[0021]FIG. 3 illustrates an alternative embodiment of a lower portion ofthe RF gas recovery system of FIG. 1.

[0022]FIG. 4 illustrates another alternative embodiment of the lowerportion of the RF gas recovery system of FIG. 1.

[0023]FIG. 5 illustrates still another alternative embodiment of thelower portion of the RF gas recovery system of FIG. 1.

[0024]FIG. 6 illustrates still another alternative embodiment of thelower portion of the RF gas recovery system of FIG. 1.

[0025]FIG. 7 illustrates still another alternative embodiment of thelower portion of the RF gas recovery system of FIG. 1.

[0026] Like reference symbols in the various drawings indicate likeelements.

DETAILED DESCRIPTION

[0027] Referring to FIGS. 1 and 2, the upper portion of an RF gasrecovery system 10 is shown for radiating electromagnetic energy into acoal seam deposited within the ground 12 and extracting gas released bythe heating generated by the electromagnetic energy. In particular, gasrecovery system 10 includes an outer casing 14 disposed within aborehole 16 drilled deep within the ground. The outer casing 14 houses acoaxial RF applicator 18 that includes a coaxial transmission line 20extending from the upper end of the antenna at the surface of the earthto a distal end of the antenna The coaxial transmission line 20 includesa center conductor 22 positioned coaxially within an outer conductor 24.In this embodiment, center conductor 22 and outer conductor 24 havediameters of about 1 inch and 2.9 inches, respectively, and have lengthsgreater than 30 feet In general, the length of the RF applicator 18 andthe outer casing 14 can be between 8 and 200 feet. Insulative spacers(e.g., Teflon) 26 are spaced along the length of the center conductor 22to maintain its coaxial position relative to the outer conductor 24.Furthermore, due to the relative long length of RF applicator 18,support collars 27 are spaced periodically along the length of centerconductor 22. Similarly, support collars 29 are spaced periodicallyalong the length of outer 24. The upper end of the coaxial transmissionline 20 is connected to an RF generator (not shown) via an RF coax line30. The upper ends of center conductor 22 and outer conductor 24 ofcoaxial transmission line 20 include expansion joints in the form ofbellows 31,32, respectively.

[0028] As shown in FIG. 2, in this embodiment, the distal end of the RFapplicator includes a dipole antenna 40 extending between 5-6 feet fromthe end of coaxial transmission line 20. In operation, dipole antenna 40receives RF energy from the RF generator via coaxial transmission line20 to and radiates the coal seam deposit in the surrounding earth. Aswill be described in greater detail below, the radiated RF energy heatsthe coal and, in particular, vaporizes or boils the water in a specifiedregion or of the coal seam. By removing the water from the coal seam,methane and other gases trapped within the coal seam are released andmore easily removed.

[0029] Center conductor 22 of transmission line 20 is dual-purposed. Thecenter conductor not only serves as part of the structure for heatingthe water in the coal seam, it also provides an inner passage 42 orconveying the gas to the surface of the earth for processing. To removethe gas, a product return pipe 44 having a removable plug 46 extendsfrom the end of center conductor at bellows 32.

[0030] RF gas recovery system also includes a thermocouple assembly 50having a thermocouple coil 52 connected to bellows 32. Thermocouplecoils serves as a filter to “choke” or prevent the flow of low frequencycurrents to flow. (Ray?) Outer casing 14 also includes input pipes 56through which nitrogen gas is introduced within the casing. The nitrogengas is much less flammable than oxygen and, therefore, provides a muchsafer safe environment for introducing high current levels from RFapplicator 18.

[0031] The operation of this particular embodiment will now bedescribed. In general, . . . RF energy is transmitted from the RFgenerator to dipole antenna 40 via coaxial transmission line 20. Dipoleantenna 40 induces currents within the coal seam

What is claimed is:
 1. A system for recovering gas trapped within theearth, the system comprising: a casing sized and configured to bepositioned within a borehole in the earth, the casing formed of amaterial that is transmissive to electromagnetic energy and gas withinthe earth; an antenna sized and configured to be positioned within thecasing, the antenna having a distal end and a proximal end andincluding: a radiating element at the distal end of the antenna which,in operation, transmits electromagnetic energy toward a desired area ofthe earth; and an interior channel for allowing gas to be conveyed fromthe distal end to the proximal end of the antenna.
 2. The system ofclaim 1, further comprising a product return pipe having a first endconnected to the proximal end of the antenna and a removable capattached to a second end of the product return pipe.
 3. The system ofclaim 1, further comprising a bellows connected to the proximal end ofthe antenna.
 4. The system of claim 1 further comprising a thermocoupleassembly connected to the proximal end of the antenna.
 5. The system ofclaim 1 wherein the antenna is configured to operate in a frequencyrange between 300 KHz and 300 GHz
 6. The system of claim 5 wherein theantenna is configured to operate in a frequency range between 1 MHz and100 MHz.
 7. The system of claim 6 wherein the antenna is configured tooperate at a frequency of about 27 MHz
 8. The system of claim 6 whereinthe antenna is configured to operate at a power level in a range between3 Kwatts and 20 Kwatts.
 9. The system of claim 8 wherein the antenna isconfigured to operate at a power level of about 10 Kwatts.
 10. A methodfor recovering gas trapped within the earth, the method comprising:positioning a casing within a borehole in the earth, the casing formedof a material that is transmissive to electromagnetic energy and gaswithin the earth; positioning an antenna within the casing, the antennahaving a distal end and a proximal end, the antenna including: aradiating element at the distal end of the antenna which, in operation,transmits electromagnetic energy toward a desired area of thy earth; andan interior channel for allowing gas to be conveyed from the distal endto the proximal end of the antenna; applying electromagnetic energy tothe antenna to radiate the earth surrounding the is casing; drawing thegas within the earth into the interior channel of the antenna at thedistal end of the antenna; and conveying the gas within the interiorchannel to the proximal end of the antenna
 11. The method of claim 10further comprising attaching a first end of a product return pipe to theproximal end of the antenna and attaching a removable cap to a secondend of the product return pipe.
 12. The method of claim 10 furthercomprising attaching a bellows to the proximal end of the antenna 13.The method of claim 10 further comprising attaching a thermocoupleassembly connected to the proximal end of the antenna.
 14. The method ofclaim 10 wherein the electromagnetic energy is in a frequency rangebetween 300 KHz and 300 GHz.
 15. The method of claim 14 wherein theelectromagnetic energy is in a frequency range between 1 MHz and 100MHz.
 16. The method of claim 15 wherein the electromagnetic energy has afrequency of about 27 MHz.
 17. The method of claim 15 wherein theelectromagnetic energy is at a power level in a range between 3 Kwattsand 20 Kwatts.
 18. The method of claim 17 wherein the electromagneticenergy is at a power level of about 10 Kwatts.